1. Field of the Invention
The invention relates to a high-frequency surgical apparatus in which, for the purpose of cutting and/or coagulating biological tissue by means of high frequency current, a high-frequency generator having a first electrode and a second electrode creates a high frequency circuit through the tissue undergoing treatment, with an electric arc being formed, having a measuring device to detect DC voltage components generated in the high-frequency circuit when the arc is formed, which DC voltage components can be used to control the high-frequency generator via a controlling device.
2. Related Art of the Invention
The invention furthermore relates to a method for operating a high-frequency surgical apparatus in which, for the purpose of cutting and/or coagulating biological tissue by using high frequency current, a high-frequency generator having a first electrode and a second electrode creates a high frequency circuit through the tissue undergoing treatment, with an electric arc being formed, and in which the DC voltage components generated in the high frequency circuit when the electric arc is formed are used to control the high-frequency generator.
Apart from some specific coagulation effects (e.g. soft coagulation, desiccation), electrosurgical instruments for cutting or coagulating biological (e.g. human or animal) tissue function by making use of the photothermal and electrical properties of alternating electric arcs which ignite and extinguish between a first electrode and the organic tissue in contact with a second electrode, in step with the AC voltage half-waves generated by the HF generator. At the start of the application, the water-containing organic tissue in the contact zone is rapidly heated up and evaporated by the direct contact with the first electrode of the instrument. The cell membranes, which are approximately 8-10 nm thick, burst open in avalanche fashion as a result of the water vapour pressure at the site of the incision. The water vapour forms a thin, high-ohmic and dielectrical intermediate layer between the electrode and the organic tissue. If a sufficiently high voltage is applied, an electrical breakdown occurs with sparks or an electric arc being formed. In the process, the mixtures of water vapour and pyrolysis gas formed by combustion are ionized and water vapour molecules are thermally dissociated. The dynamics of the ionization and electric arcing result in specifically characteristic upper harmonics and shifts in potential (faradization) in the electric circuit, which can be used to control the HF generator.
A distinction is made between so-called monopolar operation and so-called bipolar (or more generally multipolar) operation. In the case of monopolar operation, the first electrode, which is usually referred to as the active electrode and is manipulated by the operator, has a relatively small surface area, while the second electrode, which is usually referred to as the neutral electrode, is applied to the patient over a large surface area. On the other hand, in the case of multipolar operation, several electrodes having comparable surface areas are provided, such as in the case of bipolar scissors or coagulation forceps.
The arc-induced action mechanisms discussed above, combined with HF power levels high enough to vapourize tissue, not only characterize the process of surgical cutting (electrotomy) using monopolar or bipolar techniques but they are also characteristic features in hemostasis (electrocoagulation), especially in the case of contactless monopolar spray coagulation and also in the case of contactless plasma coagulation carried out under argon protective gas (argon beamer).
One problem that occurs with all types of operation is the formation of an arc between two oppositely poled metals. It may happen, for example, that the operator accidentally causes a short circuit to occur between the first electrode and a further instrument being used or between the first electrode and a metallic implant. The resulting arc formation between the metal of the electrode and the metal of the other instrument or of the implant may result in a so-called metal burn being caused that can damage the electrode, instrument and/or implant. Also, especially in the case of endoscopic applications, vapourized metal may be deposited on optical elements, thereby gradually rendering them “blind”.
DE 25 04 280 C3 discloses an HF generator control system that makes use of the upper harmonics in the current flow (harmonic component of the operating frequency of the HF oscillator), which are generated when the arc ignites and burns, as a measure of the magnitude or strength of the electric arc.
EP 0 709 065 A1 discloses an electrosurgical device in which voltage spikes introduced into the HF voltage by the formation of an electric arc are counted separately in the positive and the negative half wave of the HF voltage. This is achieved in two alternative fashions. In a first embodiment separate comparisons are performed between a threshold value and the peak amplitudes of the HF voltage on the one hand and between the same threshold value and the peak amplitudes of the inverted HF voltage on the other hand both yielding counts of voltage spikes in the positive and negative half waves respectively. In a second embodiment a high pass filter tuned to a frequency well above the HF frequency is used to allow only the very high frequency components of the voltage spikes to pass.
An alternative and less expensive means of controlling the HF generator is disclosed in DE 39 11 416 A1 as well as in the corresponding European patent application EP 0 391 233 A2. In the high-frequency surgical apparatus that is described there, which is designed for monopolar operation, control of the HF generator is based on detecting DC voltage components in the high-frequency circuit which occur as a result of asymmetrical discharge effects when the arc forms between the active electrode and the tissue in the HF circuit. The prior art high-frequency surgical apparatus has a DC voltage-measuring device designed as a discharge detection circuit, which device is connected to the electrical connections between the HF generator and the active electrode (instrument) and the neutral electrode which closes the HF circuit. The measuring device consists essentially of a current compensation choke on the output side of which a DC voltage, determined by the size of the arc, can be tapped off and then used to control the output power of the HF generator.
In the case of this prior art, generic method of controlling the electric arc, it is disadvantageous that when deviations from normal operation occur, especially when metal-to-metal arcing occurs, the controlling system fails, thereby resulting in undesirably high levels of power being applied to tissues or to the electrodes of the instruments, thus possibly harming the patients (necroses) or causing metal burn on implants and instruments.